1. Field of the Invention
The present invention relates to fingerprint detection and recognition and in particular to finger detection within integrated circuit devices employed for fingerprint-based identification and security mechanisms.
2. Description of the Prior Art
Fingerprint detection and recognition has become a widely implemented identification and security (verification) measure for a variety of applications, including credit card usage, governmental licensing and registration, and premises access control. Integrated circuit fingerprint sensors used in such detection usually contain a two-dimensional array of sensing electrodes at a pitch substantially smaller than the size of the lines on the skin surface of a fingertip, allowing an image of the fingerprint pattern to be acquired for processing, analysis and comparison.
The structure of a typical electronic fingerprint sensor is depicted in FIG. 4. Fingerprint sensor 402 includes a planar array of sensing electrodes 404 conductively coupled to detection and image capture circuitry (not shown). Sensing electrodes 404 may be covered by a protective layer 406 against which the skin surface 408 of the fingertip is placed. Ridges 410 and valleys 412 formed by the lines on the fingertip skin surface 408 are then detected utilizing sensing electrodes 404.
Fingerprint detection is most commonly achieved at least partially through capacitive coupling with the finger. In such embodiments, skin surface 408 and sensing electrodes 404 each form a capacitor having a capacitance proportional to the distance between a respective sensing electrode 404 and the overlying skin surface 408. Thus, for example, the capacitance measured by sensor 402 at a point near the top of a fingerprint ridge 410 by a sensing electrode separated from skin surface 408 by a distance dx,y (where x and y denote the position of the respective sensing electrode within the two-dimensional sensor array) will differ from the capacitance measured through another sensing electrode at a point near the bottom of a fingerprint valley 412, where the intervening distance dx,y+2 is greater. In this manner an electronic image of the fingerprint may be captured for further processing.
Fingerprint detection normally involves a relative or comparative process, in which a characteristic such as capacitance) at one sensing electrode within the array is processed relative to similar measurements by the other sensing electrodes. As a result, the fingerprint sensor is subject to attack by xe2x80x9cspoofing,xe2x80x9d or presentation of the appropriate fingerprint pattern by an unauthorized individual. For example, the simplest spoofing technique might involve use of a finger severed from an authorized individual for that purpose.
It would be desirable, therefore, to protect against unauthorized uses of fingerprint patterns being employed in order to circumvent fingerprint-based identification and security mechanisms.
Within a capacitive fingerprint detection device, finger detection is provided by a plurality of resistive grids overlying the fingerprint sensor electrodes to measure the resistance of the finger placed on the sensor surface. A finger placed on the sensor surface connects the resistive rids and allows the skin resistivity to be measured. The measured resistance is compared to a reference resistance or range of resistances to determine whether the measured resistance matches the expected bio-characteristics of living skin tissue. The finger detection thus provides anti-spoofing protection for the fingerprint detection device.